Google Maps says Kissimmee is 6 miles closer in driving distance, but driving time is only 2 minutes less because the Orlando station is close to I-4, while the Kissimmee station requires driving on local roads at least part of the way. If I-4 is backed up, the drive to Orlando would take even longer. The fastest drive to Kissimmee is using SR-417, which of course is a toll road; OTOH no tolls are required on I-4. Driving to Kissimmee without tolls makes the drive 4 minutes longer than to Orlando.
Both stations have indoor waiting rooms, ticket offices, checked baggage, and restrooms. Orlando's waiting room is quite large; Kissimmee's is small and has seating for a couple dozen people at most. But both also have benches outdoors along the platform.
Neither station has food service or rental cars on site. Sometimes there's a hot dog cart at Orlando, but not always. There are several eateries within a couple of blocks of Kissimmee if you have a longish wait, including a hot dog restaurant right next door, and the area is very walkable. I think local offices of some car rentals will pick you up at either station, but it may be limited to business hours.
Sanford station is only for the Auto Train. Regular Amtrak trains don't stop there, and it's much farther from WDW anyway.